A number of ceramic fibers and fabrics woven therefrom are a development of recent years. These fibers have provided commerce with a new family of fabrics or textiles which have a high tensile strength and modulus and the ability to maintain these properties at elevated temperatures. An inherent property of ceramic fibers, however, is their somewhat brittle nature, that is, the inability of the fiber to withstand bending stresses. When sewing thread made of such ceramic fibers is subjected to short radius stress, such as encountered in the sewing needle of machines or in the tying of knots, twisted ceramic fiber sewing thread is prone to breakage. Due to this problem, tedious and labor intensive hand-sewing has been employed to fabricate articles made from ceramic fiber fabrics or cloths that need to be sewn or tied with ceramic fiber sewing thread. As an alternative to hand- sewing, newly developed high temperature (i.e., greater than 1000.degree. C.) insulating fabrics are being machine-sewn with thread made of conventional twisted fiber construction and having a lower temperature use level than the fabric, even though such thread deteriorates at high use temperatures. Thus, there is need for a machine sewing thread which maintains its high tensile strength and modulus for prolonged periods at temperatures up to 1150.degree. C. and for at least short periods at temperatures up to 1430.degree. C., and which thread is resistant to abrasion, shrinkage, and moisture absorption, and is chemically inert.
Prior art threads lack the high temperature resistance desired in many applications. Many have organic fiber components which burn out at temperatures above 300.degree. C., resulting in disintegration of the fiber component and failure of the product for its intended use. One type of commercial fused silica sewing thread having a twisted construction begins to deteriorate at 500.degree. C. When this thread is sewn into fabrics made, for example, of alumina-boria-silica fibers (Nextel.RTM. 312), which are high temperature resistant up to about 1430.degree. C., the heat causes failure of the thread and the subsequent deterioration of the stitching. Alumina-boria-silica fibers as disclosed in U.S. Pat. No. 3,795,524 comprise aluminum borosilicate, the alumina-boria mol ratio being 9:2 to 3:15.
When damaged by abrasion or cutting, twisted thread constructions suffer from the disadvantage of unraveling. The result is a peel-back of all or part of the fibers behind the sewing needle or machine bobbin thread guide resulting in a broken or weak stitch. High modulus fibers are particularly susceptible to this problem.
U.S. Pat. No. 3,791,658 teaches a packing material for sealing movable elements of pumps, valves, and the like comprising a core of "Teflon" polytetrafluoroethylene or fiberglass asbestos impregnated with Teflon polytetrafluoroethylene and an outer tubular jacket of wrapped or braided graphite filaments. U.S. Pat. Nos. 2,649,833 and 2,712,263 teach composite strings for racquets utilizing a center core of twisted synthetic plastic filaments that is integrated with a braided jacket of plastic filaments and one or more coatings of thermoplastic material.
Composite threads, twines, and cords having core and sheath constructions are known in the art. Composite threads generally have superior specific properties over single component threads. The sheath constructions are twisted, twistless, tangled, or plastic coated strands and the core constructions are spun staple, twisted, false twisted, twistless, plastic integrated, multi-core or spaced-apart core strands. See, for example, U.S. Pat. Nos. 2,735,258, 2,861,417, 3,722,202, 3,735,579, 3,745,756, 3,807,162, 3,751,897, 3,952,496, 4,070,818, 4,145,473, and 4,176,705.